PROJECT SUMMARY Deficits in goal-directed behavior are the hallmark of many neuropsychiatric diseases. The dorsomedial striatum (DMS) has emerged as a key mediator of goal-directed actions, serving as a critical node for integration of sensorimotor, motivational, and cognitive information. Low-threshold spiking interneurons (LTSIs) exert strong inhibitory control over striatal spiny projection neurons (SPNs) and are positioned to regulate plasticity at cortico-SPN synapses through dendritic inhibition. While LTSIs are poised to play a key role in regulating goal-directed behavior, no evidence of DMS LTSI involvement in learning behavior has presently been reported. The overarching aim of this proposal is therefore to explore the role of DMS LTSIs in goal- directed behavior and examine how learning alters LTSI physiology and plasticity. The first proposed experiment investigates whether LTSIs are important for the acquisition of goal-directed behavior. To provide initial evidence for LTSI involvement in learning and behavioral flexibility, excitability of LTSIs has been reduced using cell-type specific viral expression of Kir2.1, an inwardly rectifying potassium channel, and mice subsequently trained in a goal-directed operant task. Preliminary data demonstrates that this reduction in LTSI excitability significantly enhances task acquisition. Efficient goal-directed behavior is comprised of learning action-outcome associations, selecting an appropriate action, evaluating the resultant outcome, and adjusting behavior if necessary. To temporally delineate the role of LTSIs in action selection and outcome valuation during the acquisition of an action-outcome contingency, an optogenetic approach will be implemented. Using halorhodopsin, DMS LTSIs will be selectively inhibited during the interval preceding lever choice (action selection period) or as mice retrieve and consume their reward (outcome valuation period). Prior work has shown a key role of the anterior cingulate cortex (ACC) in action selection, while the orbitofrontal cortex (OFC) has been strongly linked to outcome valuation. Interestingly, preliminary cell type-specific monosynaptic tracing data shows DMS LTSIs receive the densest innervation from the ACC and OFC. While results from the optogenetics experiment may implicate the ACC and/or OFC in LTSI function during goal-directed behavior, it will be important to more directly pursue the functional role of these circuits. As LTSIs are highly excitable and tonically active, learning may involve silencing their inhibitory effects within the striatum. This can be achieved through reductions in LTSI excitability, enhanced depression of excitatory ACC-LTSI or OFC-LTSI synapses, or reduced efficacy at LTSI-SPN synapses. The second set of ex vivo electrophysiology experiments will directly test these possibilities using projection-specific optogenetic recruitment of ACC and OFC synapses onto LTSI neurons in mice that have learned the goal-directed behavior task.